Aluminum is produced conventionally by the Hall-Heroult process, by the electrolysis of alumina dissolved in cryolite-based molten electrolytes at temperatures up to around 950.degree. C. In Hall-Heroult cells, the anodes are usually pre-baked carbon blocks that are consumed by the electrochemical reaction, corroded by contact with the electrolyte and disintegrated by the air and/or oxidizing gases present. Soderberg anodes made of a coherent carbon mass which solidifies in situ are also used.
Pre-baked anodes for aluminum production are made of a matrix of petroleum coke with pitch as binder. Their production involves various phases including preparing and treating the starting materials, mixing, forming and calcining at high temperature, followed by securing the current supply member by rodding.
The resistance of that part of the anode which remains outside the bath during cell operation is of paramount importance, not only to decrease the amount of anode consumption above the theoretical requirement but also to reduce the formation of carbon dust which is a cause of a reduction in current efficiency and an increase in cell temperature, and which must be eliminated when it collects on bath surface.
Of the several attempts to protect the anode, none has so far been satisfactory. The normal protection by aluminum spraying is costly and not always impervious. The oxidation of the carbon anodes, in the Hall-Heroult cell, outside the bath leads to a loss for the aluminum producer. Typically, instead of the theoretical consumption of 0.33 kg of carbon per ton of aluminium, often more than 0.43 kg is lost, the difference being caused mainly by air and CO.sub.2 burn.
Many elements or compounds catalyze the oxidation reaction of carbons but the inhibition of the same reaction is more difficult to achieve. In general, the oxidation reactivity of carbon is reduced with absorbers, or with ceramic protection layers. Several absorber additives have been reported, such as metal, halogen compounds, and incorporated nitrogen. Ceramic protecting layers have been proposed, formed by low melting liquid glass, such as B.sub.2 O.sub.3, Cr.sub.2 O.sub.3, silica, etc.
The oxidation prevention treatment processes contemplated for the anode can be divided into two different groups, one is an additive added after the anode baking, the other is an additive added into the carbon paste. To date, only an aluminium coating protection treatment, or a thick layer of alumina and cryolite, has worked reasonably well for oxidation protection of commercial pre-baked anodes, but has several drawbacks, such as cost and difficulties in the cell operation. No oxidation protection has so far been suggested for Soderberg continuous anodes. Several other oxidation prevention treatments have worked well in the laboratory but have fallen short of the expected performance when the same treatments have been applied to the anodes tested in commercial cells. No apparent reason has been forthcoming and the discussion of such an effect has invariably been directed towards the possibility of the composition of the anode gases being the reason for such a difference.
When boron has been added to the anode paste in the form of elemental boron or boron compound, the oxidation rate of the carbon has been reduced but the contamination of aluminum is unacceptable.
Recently, U.S. Pat. No. 5,486,278 (Mangienello et al.) has disclosed a treatment process which has been shown to significantly reduce the oxidation of the anode in the laboratory as well in commercial cell tests in the pre-baked carbon anodes. This method comprises treating the anode or other component in a boron-containing liquid to intake the boron-containing liquid to a selected depth over parts of the surface to be protected, this selected depth being in the range 1-10 cm, preferably at least 1.5 cm and at most about 5 cm, preferably still at least about 2 cm and at most about 4 cm. This method was found to significantly reduce the oxidation of pre-baked anodes in laboratory tests and in commercial test cells. However, as discussed in detail below, it has been found unexpectedly that the greatly improved oxidation resistance obtained with this treatment is partly offset by a strength loss which could lead to burn-off after a critical weight loss when the anode is subjected to stress. This could lead to problems when the components are scaled up to industrial size.
Problems like those described above for pre-baked carbon anodes apply also to the carbon cell sidewalls including a lower part submerged in the electrolyte and an upper part which is exposed to CO.sub.2 -enriched air, and which disintegrate and wear away as a result of attack by oxidizing gases.